Fractional distillation columns having a number of vertically spaced distillation trays are widely employed in the hydrocarbon processing, chemical, and petrochemical industries. Accordingly, a large amount of research, development, and creative thinking has been devoted to providing improved fractional distillation trays. Fractionation tray development has therefore provided many variations in contacting area structure, downcomer design, and overall tray structure.
Vapor-liquid contacting devices are used in a wide variety of applications for separating liquid or vapor mixtures. One of the major applications of the vapor-liquid contacting devices is in the separation of chemical compounds via fractional distillation. These devices are also used to contact a gas stream with a treating liquid which selectively removes a product compound or an impurity from the gas stream.
Within a column containing vapor-liquid contacting devices, liquid flows in a generally downward direction and vapor rises vertically through the column. On each vapor-liquid contacting device, liquid flows in a generally horizontal direction across the device and vapor flows up through perforations on the device. The cross flow of vapor and liquid streams on each device generates a froth for intimate vapor-liquid contacting and mass transfer.
The apparatus can be used in the separation of essentially any chemical compound amenable to separation or purification by fractional distillation. Fractionation trays are widely used in the separation of specific hydrocarbons such as propane and propylene or benzene and toluene or in the separation of various hydrocarbon fractions such as LPG (liquefied petroleum gas), naphtha or kerosene. The chemical compounds separated with the subject apparatus are not limited to hydrocarbons but may include any compound having sufficient volatility and temperature stability to be separated by fractional distillation. Examples of these materials are acetic acid, water, acetone, acetylene, styrene acrylonitrile, butadiene, cresol, xylene, chlorobenzenes, ethylene, ethane, propane, propylene, xylenols, vinyl acetate, phenol, iso and normal butane, butylenes, pentanes, heptanes, hexanes, halogenated hydrocarbons, aldehydes, ethers such as MTBE and TAME, and alcohols including tertiary butyl alcohol and isopropyl alcohol.
One important issue in the field of vapor-liquid contacting columns is improving the capacity of the trays to allow vapor and liquid to flow from tray to tray without flooding. A second important issue in the field is improving the efficiency of the trays for mass transfer between vapor and liquid.
In a well-known classic study by W. K. Lewis in 1936, it was found that the mass transfer efficiency of vapor-liquid contacting trays could be maximized by bringing an unmixed vapor into contact with liquid flows across each successive tray in the same direction (Case 2). The Case 2 is referred to as a parallel flow, which, as used herein, refers to liquid flows on vertical adjacent or successive trays rather than to liquid flows on a single tray. Lewis' Case 2 ensures that the driving force for mass transfer on a given tray is nearly the same regardless of where that mass transfer occurs on the tray. Because of this, substantial increases in efficiency can be obtained when using a tray operated according to Lewis' Case 2.
U.S. Pat. No. 5,223,183 to Monkelbaan, et al. teaches a parallel flow tray with at least one central downcomer and no side downcomers. The downcomers of each tray are aligned with the downcomers on the other trays of the column such that the downcomers on one tray are immediately below those on the tray above. The outlets of one downcomer are directly above the inlet of another. A pair of inclined liquid deflecting baffles over each downcomer connects the outlets and inlets of vertically adjacent downcomers and provides a crisscrossing liquid flow path. The downcomer baffles prevent liquid from the tray above from entering each downcomer and define the direction of liquid flow onto the tray deck. The inclined surface of the baffle also imparts a horizontal momentum to the descending liquid which tends to push the liquid and froth present on the tray towards the inlet of the outlet downcomer for this portion or zone of the tray. In certain designs of the trays there is provided a perforated anti-penetration weir on the lower end of the downcomer baffles, with the weir being perpendicular to the downcomer baffle. Further, froth flow into an outlet downcomer is pinched by the downcomer right above, which may reduce tray capacity.
U.S. Pat. No. 5,318,732 to Monkelbaan, et al. teaches another parallel flow multiple downcomer type fractionation tray, which increases tray capacity by providing imperforate stilling decks that extend across the tray deck surface outward from the downcomer inlet opening together with vertical inlet weirs attached to the outer end of the stilling decks. The inlet weirs may function as pre-weirs used in addition to the conventional inlet weir formed by the upward extension of the downcomer side wall. Further, the stilling decks help reduce pinching; however they also reduce the active area of the deck.
Therefore an improved high-capacity tray providing a Lewis Case 2 parallel flow pattern is needed in the art.